1. Field of the Invention
The present invention relates to a temperature switch and more particularly to a monolithic temperature switch formed by micromachining for use in applications where space is relatively limited, such as in disposable probes for aircraft windscreens, that is relatively tolerant of dynamic environments such as, vibration and acceleration forces, which includes a sensing element and a pair of electrical contacts that are controlled as a hysteretic function of temperature with a snap-acting response to minimize wear on the contacts due to contact bounce.
2. Description of the Prior Art
Various temperature sensors are known in the art. Such sensors are used in various measurement and control applications. For example, thermocouples, RTD's and thermistors are used for measuring temperature in various applications. Such sensors provide an electrical analog signal, such as a voltage or a resistance, which changes as a function of temperature. Monolithic temperature sensors are also known. For example, a diode connected bipolar transistor can be used for temperature sensing. More specifically, a standard bipolar transistor can be configured with the base and emitter terminals shorted together. With such a configuration, the base collector junction forms a diode. When electrical power is applied, the voltage drop across the base collector junction varies relatively linearly as a function of temperature. Thus, such diode connected bipolar transistors have been known to be incorporated into various integrated circuits for temperature sensing.
Although the above described devices are useful in providing relatively accurate temperature measurements, they are generally not used in control applications to control electrical equipment. In such control applications various types of temperature switches are used. Such temperature switches typically consist of a sensing element which provides a displacement as a function of temperature and a pair of electrical contacts. The sensing element is typically mechanically interlocked with the pair of electrical contacts to either make or break the electrical contacts at predetermined temperature set points. The temperature set points are defined by the particular sensing element utilized.
Various types of sensing elements are known which provide a displacement as a function of temperature. For example, bimetallic elements, mercury and reed switches are known to be used in such temperature switches.
Bimetallic elements typically consist of two strips of materials having different rates of thermal expansion fused into one element. Upon a temperature changes, unequal expansion of the two materials generally causes the element to bend in an arc. By mechanically interlocking the bimetallic element with a pair of electrical contacts, such displacement can be used to either make or break the electrical contacts.
Mercury temperature sensors consist of a mercury filled bulb and an attached glass capillary tube which acts as an expansion chamber. Two electrical conductors are disposed within the capillary at a predetermined distance apart. The electrical conductors act as an open contact. As the temperature increases, the mercury expands in the capillary tube until the electrical conductors are shorted by the mercury forming a continuous electrical path. The temperature at which the mercury shorts the electrical conductors is a function of the separation distance of the conductors.
Reed switches have also been known to be used as temperature sensors in various temperature switches. Such reed switch sensors generally consist of a pair of toroidal magnets separated by a ferrite collar and a pair of reed contacts. At a critical temperature known as the Curie point, the ferrite collar changes from a state of low reluctance to high reluctance to allow the reed contacts to open.
Such known temperature switches as discussed above are normally assembled from discrete components. As such these temperature switches are relatively large and are not suitable for use in various applications where space is rather limited. Moreover, the assembly cost of such temperature switches increases the overall manufacturing cost.
There are also various other problems associated with such known temperature switches. More specifically, many of such switches are generally not known to be tolerant of external forces, such as vibration and acceleration forces. Consequently, such temperature switches are generally not suitable for use in various applications, for example, in an aircraft. Another problem with such known temperature switches relates to the calibration. More specifically, such known temperature switches generally cannot be calibrated by the end user. Thus, such known temperature switches must be removed and replaced if the calibration drifts, which greatly increases the cost to the end user.